Clemens, in U.S. Pat. No. 3,842,194, discloses a capacitive video disc playback system which includes a high density information disc record having a conductive surface. Initially, information records for this system were sandwich-type construction, i.e. the record was formed of a plastic disc coated with a layer of conductive metal which was in turn coated with a dielectric material, such as polystyrene. Subsequently, improved information discs for this system were homogeneous and comprised a thermoplastic matrix having finely divided conductive carbon particles embedded therein.
In order to make such discs sufficiently conductive to obtain capacitive playback, a fairly high loading of carbon black is required. Formulations presently utilized to prepare capacitive information discs contain about 15 percent by weight of a low density, conductive carbon black in a poly(vinyl chloride)-based resin composition. Suitable molding compositions are disclosed in Martin et al. U.S. Pat. No. 4,228,050. Such formulations contain, in addition to the two principal ingredients, a number of additives such as lubricants, stabilizers, processing aids and the like.
The molding compositions described by Martin et al., are comparatively expensive, due principally to the amount of high quality carbon black contained therein. The number of additives present also contributes to the cost of each disc. It would be beneficial to be able to significantly reduce the amount of carbon black and additives present in the disc without losing the conductivity necessary for capacitive playback of high quality video and/or audio signal information.
Another significant expense in the manufacture of capacitive electronic discs is the cost of stampers. The discs are presently produced by compression molding. Considerable flow of the carbon black filled composition takes place in the mold under heat and pressure, and the presence of large numbers of filler particles in the molding composition causes stamper scratching. Because the information pattern embossed into the disc surface is extremely fine, scratching of the surface of metal stampers ends their useful life.
A potential means of reducing the carbon black content of a capacitive electronic disc would be to produce a disc having a less expensive plastic central core disc with conductive regions or layers on one or both surfaces. O'Mara, in U.S. Pat. No. 4,390,487, discloses a method of forming such a layered capacitive electronic disc in which the conductive plastic is injected into a compression mold and a nonconductive core material injected inside the conductive material to form a layered preform which is then compression molded to form a capacitive electronic disc. Discs made by this process contain up to about 70 percent by weight of nonconductive material. While this approach reduces costs by reducing the amount of carbon black required, the discs are still formed from a preform or puck. Therefore, the problem of stamper wear is undiminished.
Ruda, in copending U.S. patent application Ser. No. 522,332, filed Aug. 11, 1983, discloses a layered-type capacitive electronic disc having very thin conductive layers in comparison to those in a disc prepared according to O'Mara. In Ruda, the conductive plastic composition in molten form is made into sheets about 2 to 30 mils thick by passing between rollers in production calendering equipment or in a two-roll mill. Layered discs are formed by compressing a core disc of a conventional nonconductive formulation between two such sheets. This method addresses both of the above problems. However, such a layered disc has the potential for delamination of the conductive surfaces. In addition, there is the added problem that imperfections in the surface of the core disc may "print through" into the information pattern thereby causing distortions in playback.
Dixon et al., in copending U.S. patent application Ser. No. 556,354 filed on Nov. 29, 1983 and Di Marco, in U.S. Pat. No. 4,515,830, disclose processes of forming a layered or sandwich-type capacitive electronic disc utilizing a dispersion of conductive carbon black particles in a solution of a poly(vinylchloride)-based resin. The dispersion is either coated onto a core disc or onto a support surface and dried to form a conductive layer. The layer formed on the support surface is thereafter laminated onto the core disc. The desired information is then embossed into the conductive layer. Although the techniques described by Dixon et al. and Di Marco produce very thin conductive layers, they require the handling of organic solvents with the environmental problems associated therewith.
In accordance with this invention, a method has been found to prepare capacitive electronic discs which is advantageous over previous methodologies.